The present invention relates to a coating apparatus to apply to a travelling web, particularly a paper web, size or coating agent, as wellas a process for such coating of a travelling web.
The problems with traditional methods for the coating of travelling webs, for example the coating of paper webs, are associated with requirements for increasing web speeds. Such high web speeds result in problems when for example a paper web is fed into a nip between a support roll and a dosage roll and the coating composition shall be transferred from the dosage roll to the paper web in said nip. When the paper web at speeds which can reach 2000 m/min is fed into the nip an air film accompanies the pader web. This air film forms a barrier preventing the liquid transferred by the dosage roll to come into contact with the web in a uniform manner. The presence of this air film furthermore results in problems in that the coating agent is mixed with air thereby forming an unhomogeneous film, certain sections of the web not being covered with coating agent. The downstream evening member, for example a blade, can then not always even out the coating agent so as to cover said sections but uncoated sections remain even after the evening member. Such coating defects are called xe2x80x9cskipsxe2x80x9d in the parer industry and result in uneven coating.
In order to reduce the problems to a reasonable level it is required that the rotational speed of the application roller and the nip between the opposite rolls have the correct magnitude. It is found in practice that the peripheral speed of the dosage roll is of the order of 150 m/min and that the distance between the dosage roll and the support roll is within the range about 0.5 to 0.1 mm. Under these conditions coating can be obtained without skips arising in the coating. However, the adjustment of speed and play in the nip result in substantial problems in connection with practical operation. The sensitivity of the system means that the possibility of controlling the metered amount of coating agent by controlling the distance between the rolls is lost.
Practical experience shows that the quantity of coating agent which must be metered for resulting in an even coating is constituted by a very large excess, of the order of 15 to 30 times the quantity finally deposited on the web. This means that large amounts of coating agent have to be pumped and recirculated which results in cost and space problems in the pumping and sifting of the coating agent.
It should be noted that the present invention is not restricted to solely the coating of paper webs but can be used for the coating of other materials, for example for sizing on travelling paper webs, but the invention will in this disclosure be illustrated mainly in connection with the coating of paper webs.
In summary, it is thus noted that today""s techniques for sizing or coating of travelling webs is associated with the following problems.
The formation of an air barrier adjacent to the web results in the generation of so called skips.
Substantial admixture of air in the coating agent takes place in connection with the dosage nip.
There is restricted possibility to control the metered quantity.
This means that a large excess has to be metered which in turn results in the following disadvantages.
Large return flows from the evening member result in significant admixture of air and splatter.
Parts of the metered film leaves the support roll (by centrifugal force) and results in further splatter and admixture of air.
High impulse forces are generated on the member, for example a coating blade, which provides for final evening.
The demand of a large excess of coating agent results in requirements for pumps and sifts of high capacity.
Accordingly, the main object of the present invention is to provide an application apparatus for the coating of a travelling web, which apparatus enables more freedom in the control of the metered quantity of material while providing for an even coating.
Another object of the invention is to provide an application apparatus whereby the admixture of air in the material applied is reduced to a minimum.
Yet another object of the invention is to provide an application apparatus enabling substantial reduction of the excess of metered quantity of sizing agent or coating agent.
A further object of the invention is to provide a process for the application of sizing agent or coating agent on a travelling web while obtaining even coating and avoidance of the generation of so called skips.
For these and other objects which will be clear from the following disclosure there is provided an application apparatus for the coating of a travelling web comprising a support roll and a predosage roll, said rolls forming a nip between themselves through which the web is brought to travel. The application apparatus further comprises an application device by which sizing agent or coating agent can be transferred onto the web. The application device also includes means for final evening or levelling of material applied onto the web.
In accordance with the invention this application device is designed in such a manner that it transfers directly onto the travelling web said material in a certain excess before entering the nip where predosage is provided before the subsequent and final evening of applied material. By this arrangement the travelling web provides for feeding of the applied material up to the nip between the support roll and the predosage roll, the latter having a relatively low rotational speed in the same direction on the paper web. This rotational speed for the predosage roll is not critical in the same manner as in the prior art in view of the fact that the roll in question no longer has for its function to transport said material up to the nip. In view of the fact that the air film on the travelling web is broken in view of the design of the application apparatus a homogeneous wedge of material is now formed in the entrance to the nip between the counter roll and the dosage roll, and the excess from this wedge flows along the peripheri of the predosage roll down into some suitable collection means further described below. In this manner the distance between the two rolls can now be used as an instrument for the control of the flow of applied material up to the member for the final evening.
According to a preferred embodiment of the apparatus according to the invention said application means can comprise a tube extending across substantially the whole width of the web and which is provided with a slit extending along the tube or apertures or nozzles distributed along the tube. Through this slit or through these apertures or nozzles sizing agent or coating agent can be metered out and directly applied to the travelling web.
With regard to the position of the application means it is suitable that it is arranged in such a manner that. the application onto the travelling web takes place within the area where the web engages the support roll and particularly is positioned upstreams of the nip within a sector angle of up to about 30xc2x0. The lower limit is not particularly critical but it should be ensured that all coating agent reaches the web before the nip.
The application apparatus according to the present invention is suitably provided with a collection tray intended for gathering excess of sizing agent or coating agent originating from the nip and preferably also from said evening member. It is suitable in this connection to arrange the tube of the application means rotateable so that with interrupted coating of sizing agent or coating agent the tube can be turned to a position in which the material is discharged into said collection tray. This avoids the disadvantage residing in a need for interrupting the supply of material to the tube at minor shut downs. Such an interrupted supply of material to the application means can result in problems in the form of blocking of the metering slit or the apertures or other disadvantages.
In a particularly preferred embodiment of the application apparatus according to the present invention the predosage roll is on its mantle surface provided with circumferential, mutually parallel rifles or grooves. By this design a more accurate predosage of material deposited on the travelling web can be obtained. In connection with this embodiment the predosage roll is suitably provided on its mantle surface with a close wire winding, the individual wire turns pairwise forming said groove or rifle. As an alternative to such wire winding the roll can be provided with milled or cut grooves. With regard to details concerning these later embodiments reference is made to the Swedish lay-open print 463 078, the disclosure of which is introduced herein by reference.
An alternative preferred embodiment of the application apparatus according to the invention can be designed in such a manner that for the purpose of providing a contact surface which is substantially constant across the whole width of the web, the predosage roll is warped and forms an angle to the support roll. With regard to details concerning this embodiment reference is again made to the above Swedish lay-open print.
The member for final evening of the material supplied onto the travelling web can be designed in different ways and can for example be constituted by a kind of doctoring rib, and it is particularly preferred that said member comprises a conventional coating blade which is suitably arranged on a blade holder.
The present invention also provides a process for the application of sizing agent or coating agent on a travelling web, particularly a paper web, as for coating thereof. This process includes predosage and final evening of the coated material and is characterized in that the material is directly applied to said travelling web in excess and then predosed in connection with the passage of the web through a restriction zone for further transfer to a final evening zone.
From the above disclosure it is clear that the invention provides an application apparatus where the predosage roll contrary to the prior art is not used for the take up of sizing agent or coating agent by being immersed therein, and in addition to the fact that the disadvantages of the prior art as stated earlier are eliminated also other advantages are gained. Among these there may be mentioned that since the predosage roll is not immersed in the material to be applied the collecting tray can be made simpler and cheaper and, in addition, the risk for sedimentation in the tray is eliminated and the cleaning of the tray simplified.